[KUDU-810] Add "UNRECOGNIZED" value 0 for all protobuf enums

• Type: Bug
• Status: Resolved
• Priority: Blocker
• Resolution: Fixed
• Affects Version/s: Private Beta
• Component/s: cfile, client, tserver
• Target Version/s: Feature Complete
• Assignee: Dan Burkert
• Reporter: Todd Lipcon
• Created: 09/Jun/15 15:30
• Updated: 15/Jun/15 19:15
• Resolved: 15/Jun/15 19:15

Description

If you use an old build and try to deserialize a PB written by a new build, and the PB uses an enum value that isn't valid in your old build, PB defaults to setting it to the first-listed protobuf value.

We should ensure that the first value, then, is UNRECOGNIZED or UNKNOWN. Otherwise, we risk silently thinking we have valid data which makes the error much harder to debug.